A mass spectrometer called the “Q-TOF mass spectrometer” is commonly known as one type of mass spectrometer. As described in Patent Literature 1 (or other documents), a Q-TOF mass spectrometer includes: a quadrupole mass filter for selecting an ion having a specific mass-to-charge ratio from ions originating from a sample; a collision cell for fragmenting the selected ion by collision induced dissociation (CID); and a time-of-flight mass separator for detecting product ions generated by the fragmentation after separating those ions according to their mass-to-charge ratios. As the time-of-flight mass separator, an orthogonal acceleration time-of-flight mass separator is adopted, which accelerates ions in an orthogonal direction to the direction of the injection of an ion beam and sends those ions into the flight space.
In the time-of-flight mass separator, if a flying ion comes in contact with residual gas, its flight path changes, and its time of flight also changes. Consequently, the mass-resolving power and mass accuracy become lower. To avoid this problem, time-of-flight mass separators are normally placed within a high-vacuum chamber maintained at a high degree of vacuum (on the order of 10−4 Pa). On the other hand, the collision cell for dissociating ions are continuously or intermittently suppled with CID gas, and this gas leaks from the collision cell. Therefore, the collision cell cannot be placed within the high-vacuum chamber in which the time-of-flight mass separator is located; the cell is placed within a medium-vacuum chamber which is separated from the high-vacuum chamber by a partition wall and has a higher level of gas pressure than the high-vacuum chamber. The product ions generated within the collision cell are transported into the high-vacuum chamber through an ion-passage hole formed in the partition wall separating the medium-vacuum chamber and the high-vacuum chamber. The ion-passage hole needs to be extremely small to maintain the degree of vacuum within the high-vacuum chamber. In order to make ions efficiently pass through such a small hole, an ion transport optical system for transporting the ions while shaping the cross-sectional form of the ion beam is placed between the collision cell and the partition wall.
A representative example of the ion transport optical system used in a mass spectrometer is a radio-frequency multipole ion guide disclosed in Patent Literature 2 (or other documents). A radio-frequency (RF) multipole ion guide is a device for transporting ions while oscillating the ions by a radio-frequency electric field in such a manner as to confine the ions within a specific space surrounded by a plurality of electrodes. In the case of an ion transport optical system which is placed within the medium-vacuum chamber due to the CID gas supplied to the collision cell as noted earlier, the collision of the ions with the gas must be considered. The collision of the ions with the gas produces a cooling effect which deprives the ions of energy. This cooling effect favors the converging of the ion beam in the RF multipole ion guide which traps ions by a radio-frequency electric field. In other words, the RF multipole ion guide is suitable for converging ions ejected from the collision cell and guiding them into the micro-sized ion-passage hole within the medium-vacuum chamber maintained at a comparatively high level of gas pressure. Therefore, in conventional Q-TOF mass spectrometers, RF multipole ion guides have been commonly used as the ion transport optical system located between the collision cell and the partition wall within the medium-vacuum chamber.
On the other hand, the ion transport optical system located between the partition wall having the ion-passage hole and the orthogonal accelerator of the time-of-flight mass separator within the high-vacuum chamber is primarily used to produce the effects of shaping the cross-sectional form of the ion beam as well as adjusting the kinetic energy possessed by the ions. These effects are essential because, if an ion with a large amount of kinetic energy is allowed to enter the orthogonal accelerator, the ejecting direction of the ion from the orthogonal accelerator may become excessively tilted from the orthogonal direction and cause the ion to miss the detector after passing through the flight space. Unlike the medium-vacuum chamber, the contact of ions with the gas barely occurs within the high-vacuum chamber, since there is practically no residual gas in this chamber. The ion-cooling effect by the collision with the gas will not occur, and the trapping of the ions by a radio-frequency electric field will scarcely work insignificantly. Therefore, in many cases, an electrostatic ion lens which controls the trajectory and kinetic energy of the ions by a DC electric field is used as the ion transport optical system located within the high-vacuum chamber.
Other than the Q-TOF mass spectrometer mentioned earlier, there are some types of mass spectrometers constructed as a differential pumping system for transporting ions from a medium-vacuum region of approximately 1 Pa to a high-vacuum region through an ion-passage hole formed in a partition wall. For example, the configuration of a differential pumping system similar to the Q-TOF mass spectrometer is adopted in a mass spectrometer in which an atmospheric pressure ion source, such as an electrospray ion source, is used as the ion source of a time-of-flight mass spectrometer. Another example is a Fourier transform ion cyclotron resonance mass spectrometer, in which residual gas may possibly produce adverse effects on the performance of the device, as in the case of the time-of-flight mass separator. Those types of mass spectrometers also commonly use the combination of a RF multipole ion guide located within a medium-vacuum region on the front side of a partition wall and an electrostatic ion lens located within the high-vacuum region on the rear side of the same wall, to transport ions across the two vacuum regions with different degrees of vacuum.
The RF multipole ion guide located within the medium-vacuum chamber or medium-vacuum region can transport ions with a high level of efficiency. However, it has a large number of electrodes, and those electrodes need to be shaped and arranged with a high level of mechanical accuracy. Furthermore, the voltage source for applying voltages to the RF multipole ion guide is complex in configuration, since there are complex conditions concerning the voltages individually applied to the electrodes. Due to these factors, RF multipole ion guides are normally far more expensive than electrostatic ion lenses.